1. Field of the Invention
The invention relates to a method and a plant for reheating flue gas behind a wet flue-gas desulfurization plant.
2. Description of the Prior Art
In known plants for wet flue-gas desulfurization of flue gas containing gaseous sulfur compounds, the flue gas is cooled down to a flue gas exit temperature of about 45.degree. C., depending on the method. This temperature is distinctly lower than that temperature which is indispensible for maintaining the required buoyancy in the chimney. It is also below the dew point. The purified flue gas, also called "pure gas", which flows out of the flue-gas desulfurization plant carries with it small acidulated water droplets which lead to corrosion in all following components.
In order to improve the buoyancy in the chimney and to evaporate the water droplets in the flue gas it is known to reheat the purified gas flowing out of the flue-gas desulfurization plant at a temperature of about 45.degree., with energy from an external source. In this process considerable sums must be spent for energy cost.
It is also known to heat the purified gas flowing out of the flue-gas desulfurization plant by bypassing part of the hot undesulfurized flue gas which normally flows through the flue-gas desulfurization plant and mixing the hot bypassed gas with the purified gas which leaves the flue-gas desulfurization plant. This method can be performed at relatively low costs. However, it leads to SO.sub.2 emission values which are above the permissible emission value for most coal types.
A plant for reheating flue gas behind a wet flue-gas desulfurization plant has also become known, in which the flue gas is first conducted through a flue gas-air preheater, subsequently to a flue gas dust separation plant and, before it is conducted into a flue-gas desulfurization plant, through a raw gas/purified gas heat exchanger. In the latter, it is first cooled down to about 60.degree. to 80.degree. C. and, after the flue gas sulfur separation, is reheated to about 90.degree. C. This plant operates with only small thermal losses and also generates a sufficiently strong buoyancy in the chimney. However, due to the fact that the temperature is below the dew point, corrosion and contamination in the raw gas/pure gas heat exchanger is considerable.
It has also been proposed to conduct the flue gas through a flue gas air preheater, an electrofilter, a cold air preheater into the flue-gas desulfurization plant and to admix part of the fresh air which had been preheated in the cold air preheater and then in the flue gas/air preheater to the purified gas flowing into the chimney. This plant, which brings with it good utilization of the thermal energy of the flue gas, requires considerable investment for the fabrication of the different additional components. Because the temperature is below the dew point, considerable corrosion must be expected here in the cold air preheater.